Feasibility Study On Use of Gafchromic EBT2 Film as An Absolute Dosimetry QA System for SBRT Treatment

W Duggar*, C Yang, University of Mississippi Med. Center, JACKSON, MS

Purpose: To establish the use of Gafchromic EBT2 film as an accurate and effective QA tool capable of providing absolute dosimetry for SBRT treatment plans.

Methods: A dose calibration curve using the absolute dose response of Gafchromic EBT2 film after irradiation of known doses from 0 - 2500 cGy was created and used to compare planar doses created by the Pinnacle treatment planning system to EBT2 film dose responses after being irradiated with the patient's plan on a QA phantom of solid water. FilmQAPro 2012 software was used to compare the calculated and measured planar doses in absolute dose mode. Both manual and fiducial marker registration methods were performed. For manual registration, the film was adjusted using the software's movement tools in order to get the best agreement between the film and calculated planar dose. Intended as the primary method, fiducial marker registration involved marking the isocenter on the film at the time of plan delivery and then setting these marks to the isocenter of the calculated planar dose before comparison.

Results: Two cases were chosen: a single arc that could not be measured with the Delta4 dosimetry system despite troubleshooting and another composite plan involving a non-coplanar arc. In both cases, the measured and calculated planar dose comparisons had a ?-index <1 when using ± 3% dose and ± 3mm DTA criteria for above 95% of the points for both relative and absolute registration without dose rescaling.

Conclusions: Initial results show that EBT2 film can serve as an adequate QA tool for the evaluation of patient specific QA involving SBRT. We found that film offers the superior spatial resolution needed for SBRT cases which involve extremely steep dose gradients and small areas of dose deposition. Additionally, film also performs well with SBRT plans involving non-coplanar beams or arcs.